Gold-Prussian blue nanocomposite modified ITO for cysteine sensing

The transition metal hexacyanometallates form an important class of the insoluble mixed valence compounds. Prussian blue (PB) is a classical prototype of metal hexacyanoferrates with well-known electrochemical [56,57] and potential analytical applications [58,59]. The structural aspects and electrochemistry of PB have been previously reviewed in several articles [60]. As the transition between PB and its reduced form, Prussian white (PW) is associated with relatively faster electron-transfer kinetics and serves as the redox mediator in catalysis and electroanalysis [61]. The CV of the modified electrode revealed the classic and today well-known form of the reversible reduction and oxidation of PB.

The traditional synthesis methodology of PB and its analogues with the general composition K3Fe(CN)6 is based on a direct precipitation reaction of the potassium cations and the Fe(CN)63- anions in a neutral aqueous solution. A versatile method for the preparation of Prussian Blue modified electrodes based on a simple electrochemical reduction of a

ferric–ferricyanide solution was addressed [62-64]. It is well known that PB has a small solubility product constant, indicating that upon direct mixing Fe²⁺ coordinates immediately with Fe(CN)63- to form PB. PB has been prepared by both chemical [65] and electrochemical [66,67] methods.

A thin surface derivative layer of PB often imparts fast electron-transfer characteristics to the substrate. This electrocatalytic phenomenon is useful from both analytical (i.e., sensor) and electrosynthetic viewpoints. PB and its analogues have been considered as another class of interesting materials for electrocatalysis of reduction of hydrogen peroxide (H2O2).

Through its redox states, the PW state has been recognized as reducing H2O2 and acting as an electron-transfer mediator between the electrode and H2O2 for its reduction to occur at low potentials. To improve the traditional synthesis methodology and using a slow reaction process, the synthesis of PB film with Au nanoparticles by CV was used. Even though PB formation can take place on Au from a single ferric cyanide solution under low pH conditions [68-70], the growth was found to cease after a few number of cycles. When ferric cyanide solution is used in an acidic medium for potential cycling, the only solid phase that is formed on the electrode surface is PB, because the acid medium promotes release of ferric/ferrous ions that can combine with the solution ferric cyanide. In this approach, the mixture solution of HAuCl4 and ferric cyanide was used for the formation of gold- Prussian blue (Au-PB) nanocomposite films. The advantage of the interactions of HAuCl4 and ferric cyanide such as (a) Au-catalyzed decomposition of ferric cyanide under low pH conditions for the supply of ferrous ions; (b) low interfacial pH arising from aqueous HAuCl4 and (c) anodic oxidation of Au are contributed for the modification [71].

For these applications PB modification has been fully tested with many electrode materials such as glassy carbon [64] and graphite [72]. In order to improve the application, a disposable material as the base substrate was employed. Recently, an application of PB with screen-printed electrodes (SPEs) which has been widely used as a disposable electrode

substrate has the advantages of low cost and easy operation [73,74]. Indium tin oxide (ITO) is another possible material that has recently become widely used industrially for thin display panels; thus, it is easy to purchase at standard specifications [75]. ITO is an excellent

photoelectric material because of its high conductivity and photo-penetrability. Although it can decrease electrical resistance when used as an electrode substrate, the electrocatalytic activity of ITO is relatively low when compared with most other types of detectors.

Figure I-6. The Au-PB films were formed by CV behavior of the ITO electrode on potential cycling.

Surface modification of ITO glass for improving the electrocatalytic activity of electrode by carbon nanotubes (CNTs) was published [51]. In this study, the ITO glass was employed as a base substrate for the modification of Au-PB films. The CV behavior of the ITO electrode on potential cycling to form the Au-PB films is shown in Fig. I-6. The structure of the Au-PB nanocomposite modified on ITO electrode was characterized by SEM as shown in Fig. I-7.

The Au-PB nanocomposite structures are getting more and larger with the concentration increasing of ferric cyanide containing HAuCl4.

Figure I-7. SEM micrographs of the surface on (a) bare ITO and (b) PB modified on ITO; the Au-PB nanocomposite made by (c) 1 mM HAuCl4 and 1 mM K3Fe(CN)6; (d) 2 mM HAuCl4

and 2 mM K3Fe(CN)6 on ITO.

Cysteine, an important sulfur-containing amino acid in living systems, its deficiency is associated with a number of clinical situations, such as liver damage, skin lesions, and slowed growth [76,77]. A variety of chemically modified electrodes have been investigated in order to enhance the analytical signals. The modification layer on the electrode surface generally involves electron-transfer mediators, and thiol oxidation can be facilitated through

electrocatalytic conversion. The direct electrochemical determination of cysteine in real biosamples has been less-often reported, mainly because of substantive interference. To achieve this attractive goal, the detection selectivity has to be greatly improved. In the present report, the electrochemical oxidation of cysteine at an Au-PB nanocomposite films modified ITO electrode was studied. The continuing amperometric sensing result of cysteine is shown in Fig. I-8.

Figure I-8. Amperometric response of the Au-PB nanocomposite modified electrode by adding different concentrations of cysteine (as shown inset figure) at 1.2 V (vs. Ag/AgCl) in 0.1 M pH 2.4 PBS.

This method offers simple procedure to fabricate the nanocomposite film containing the properties of both gold and PB. The sample was simply to obtain a steady state current and the concentration determined from a calibration graph. It was found to be specific for cysteine oxidation and did not interfere with the matrix in human urine sample at an operating

potential of amperomeric sensing. In addition, chronoamperometry was applied to measure the cysteine concentration in the urine samples following a simple step and was gotten the cysteine recovery from urine sample more than 94.8%. Without complex electrochemical analytical method and any sample treatment procedures, this work provides efficient method for determination of cysteine in real biosamples.

I.2 The measurement of ethanol based on disposable platinum nanoparticles modified